Maybe their GPS didn't work at first. It's not an API problem.

I have the original GPX track. It is my own track I am using for testing purpose. The Strava stream missing 71 points at the start. The “time” stream start at second 71. The Garmin Watch setup to taking points every second.

I have been trying a couple of other tracks. The problem seems to be the same af all. It varies from 40 to 75 missing points in the start og the stream and nearly the same in the end. I have tried both resulotion "high" and no resolution.
It is import for me all the points are there. It will be used in a time registration app, where, among other things, it must be verified that the runner has passed the starting point.

What scopes did you request/grant when doing the oauth flow? If you only have activity:read the data from Strava will have privacy zones applied which would explain missing data at the start. You need activity:read_all to get the full data stream.